Any tech. guys in here?

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no no,the only reason i would change the connecting rod is to change the port time area,this is not the same as the port timing.Changing the port timing would be done by cutting the port roof higher or lowering the cylinder by machininge the base.To high of a transfer port can kill your rngine power(depending on your whoel setup,what your building it for ect.)if you don't have enough blow down time.Blow down time(the amount of degrees in crank rotation the exhuast is open before the transfer ports open)

What kind of compression do these things normally run?
 
with these motors the contecting rod has little to do with timming and so on, im trying to keep this simple as im not good with words, but its all to do with the pistons, ports and trasfers.

29mm and 30m cranks will change the timming, but normaly not for the better unless the head has been machines for the longer stroke crank, sure you can stack gaskits and they engines work very well, but in a perfect world, the head needs to be machines to run its best so the timming and so on isnt out or wack.

i will sugest read up on how piston ported 2 strokes work, and about pipes and exhaust pulse work and so, its has more to do with power and timming then contecting rods so to speak

as i have said befor the best way to change timeing is to change the compresion, more compresion will change the timming, and the transfers and ports will close of quicker and the extra compresion, with give more power thru the power band.

its hard to get right into and in still learing but over the last couple of years and many many motor builds ive found changing compression, changes time and changes the performance.

you can go right down to 0.4mm 0.016 thou, this will change timming and increase power and heat any lower and things will crack.

or mabe you can buy a ported modifed head check it out and compare it to a stock head and you can measure it and see whats been done to improve performace

its very late hear ill try a reword it tomorow after a good sleep
 
C.W.Hutzell said:
no no,the only reason i would change the connecting rod is to change the port time area,this is not the same as the port timing.Changing the port timing would be done by cutting the port roof higher or lowering the cylinder by machininge the base.To high of a transfer port can kill your rngine power(depending on your whoel setup,what your building it for ect.)if you don't have enough blow down time.Blow down time(the amount of degrees in crank rotation the exhuast is open before the transfer ports open)

What kind of compression do these things normally run?
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i know whats you mean but its all to do with machining and the trasfers/porting

when i talk compreasion theres many ways to do it, machine the inside top of the head.

machines the bottom, where it bolts onto the crank case

and machine the piston

or change the head gaskit size, the easyer and very common way
 
if you look at all the top european engines builders and the top usa engines builders and the top australian engine builders (europe being the better it seams)

they don't change the contecting rods, its all about porting and macining, the heads and pistons and or even read case or read valves with porting to suit

then to build larger cc engines with more torque you use longer stroke cranks like 29mm and 30mm with machined heads.

i know excatly what you mean, but just look around and see what the top guys are doing, thye have been doing it for many years and have poven what they do to work.

it will help to start with a decent engine, a 30cc clone motor will give you bugger all revs and power, its also hard to give advice not even knowing what egine or what kind of rc the engine will be used for
 
im going to go look at 338's motor today at the track.Ill have all my questions answered when i take a look at it.From all the "ported" cylinder pictures ive seen not much has been done to the actual transfer windows at all,most of the work is done to the tunnels and the exaust port.
 
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I would say your more advanced in to engines..than most people in the forum
if your using acl & meth for drag engine's with the carbs....
as for compression these little RC 2 stroke are about 120 psi with new piston ring & cylinder
Dry measure.....As benckie pointed out..the cranks come with con rods with differemt
mm's added to the con rod...28mm +1 =29mm..+2 =30mm...that's about as tech as you will
get from most hobby people....You will have to seek out a pro engine builder...
As too porting tha is not much material take off the cylinders...some minor window
changes and work on the transfer's...mostly exhaust window by 2* say you have 178*
degrees you go to 180 * degrees..Not much of a change...Best to you on your quest..
you will find the pum 260 sikk cylinder are thicker walls and are proted different
than the other cylinders....Depends what you are after..high rpm's or torque....
you want a all out race engine..go to Germany buy a CAD 5 engine from rc-on line
....Slawhammer.....
 
i went out today to look at 338's stuff.Man there little things,lol.From what i come up with the connecting rods are around 43mm on the centers,the crank was still in the cases so it was kinda hard to get an accurate measurment,but that should do me for now.Im suprised at what some guys told me as far as what RPM they turn,some of the nitros turn 40 to 60grand.I think making big horsepower would be kinda pointless for one of these because they already have enough HP what thye need is more torque to get them from a start quicker.Thanks for all the help guys
 
C.W.Hutzell said:
im going to go look at 338's motor today at the track.Ill have all my questions answered when i take a look at it.From all the "ported" cylinder pictures ive seen not much has been done to the actual transfer windows at all,most of the work is done to the tunnels and the exaust port.

As far as the compression i know how to achieve that,the enignes that i build are all custom,very technical engines mostly methanol drag engines and some MX engines(thats probibly why i sound stupid wanting to know con rod length).I just dunno what kind of compression the rc guys usually run(in psi ex. XXXpsi)I thought about just buying a race motor and just tearing it down and looking at it
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most porters will not show actual pictures of there ported heads, i know some guys who don't want there ported heads posted on the net.

i know excatly where your comming from with conecting rods.

as for compression, i think most including my self go by squish, the gap between the out edge of the piston to where it meats the head, this gap, or squish should be around 0.016 to 0.028 the lower it is the more compresion this also will effect timming so to speak and give more power across the band.

so i cant give you a psi reading, as i go by squish, my current motor is a mild ported 26cc with a squish of 0.5mm 19 thou you don't want to go lower then 0.4mm 16 thou as things tend to bend and crack.

as for torque this can be improved by porting also, and it also depends on witch motor you use, a stock 4 bolt g240rc or a g270rc will have higher torque then a g230rc and a g260rc, but the g260rc will rev higher then a g270rc.

then you can add +1 or +2mm cranks this will add more torque, less top end revs but they will handle higher gearing better so take will not be affected and top speed will be improved.

but generaly speed comes from top end revs the higher they rev the faster they will go.


C.W.Hutzell said:
i went out today to look at 338's stuff.Man there little things,lol.From what i come up with the connecting rods are around 43mm on the centers,the crank was still in the cases so it was kinda hard to get an accurate measurment,but that should do me for now.Im suprised at what some guys told me as far as what RPM they turn,some of the nitros turn 40 to 60grand.I think making big horsepower would be kinda pointless for one of these because they already have enough HP what thye need is more torque to get them from a start quicker.Thanks for all the help guys
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i persoanly feel more then 4 hp makes most 2wd cars a handful a 3 hp motor and a good pipe on a baja 5b is more power then you can use, im using a 4.5 + hp set up on a fg mt and its fricken nuts and im hitting a good 50mph, i wont break any speed records, but 50 mph in a monster truck is faster then what its made for.

ive got some one working on a new head for me. fully ported machined head and machined, ported lightened piston with a good pipe im thinking aound 6 hp and huge revs but i think the fg mt will be a handful and the motor might suit a boat better as i know i will not be able to get the power to the ground and the fg mt will just wheelie
 
yes they do. check out the Trevor Simpson Reed Cases for them. anything from a 23cc to a 30.5 cc topend will work, but they need to be ported for the reeds.

Killer power bands on these TS cases. but they arnt cheap, but SOOOO worth the cost of admission.

320 for the 2 bolt or the 4bolt case.
 
I can testify to squish clearance, Zen recommend no less than .017, I've run .015 with no bother, i tried to push the envelope last time in my ONB ported 28.5 and ended up smoking it, got too much heat in the cylinder and piston on the exhaust side and it seized solid just below the ex port, mixture was perfect, the flame pattern on the piston crown was now butterfly shaped with a bigger wing (so to speak) on the exhaust side...i'd recommend .020 squish for trouble free running.
 
thats pritty tight on the squish,i think if this is a high rpm engine i would loosen it up to .020 and go maybe 2 degrees greater on the squish angle then the piston crown angle
 
im running 0.5mm on my ported zen for my monster truck 19 thou and it goes well.

ive gone down to .4mm 16 thou and engines run good, any smaller and the heads normaly crack, the engines are hard to start and the piston pins bend

most good engine buildes will say 16 thou is the lowest squish,ive noticed the squish changes fron head to head, they are all not 100% the same.

so to be on the safe side use 0.20 (stock gaskits as the motors squish can be from 16 thou to 28 thou, most ported heads will have more compresion, so there no need to use a smaller gaskit, unless told to do so.

this is a stock cy 28.5cc head use with 30mm crank and 0.60 head gaskit, there was still to much compresion, lots of low power but with in 3 tanks it faild

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wow! that's an engine failure to be proud of. Only other one i see like that was our local LHS ownwer who flipped his 5b into a deep puddle wie a hot motor, instant crack.
mine got a hot spot underneath the exhaust port, and seized so bad i had to smack it out with a bar and 24oz estwing:hammer:, huge melted spot on the side of the piston near the top, not pretty
 
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that head was just one of thoese things, it also cracked thru the exhaust i used 0.60 head gaskit cause of the 30mm crank, and it just whent like that in 3 tanks, it kept runnign form a while if you held full wot, the baja was never flipped or crashed and was only driven on flat ground due to run in and the carb settings where stock.

ive had outher motors go like that when trying to get maxium compression out of them and ive gone to far, an they crack in the same spot and or bend the piston pin bearing, and ive had heads go like that with side pipes and bad crashes on the baja 5b

i poste this up to show all engines squish can be difrent and it pays to check even stockers, just incase as ive found 8 thou pluss difrence in some heads, so one motor you can get away with a 0.12 gaskit, the next motor you cant caus eitas like putting a 0.04 head gaskit on it is out
 
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